Measurement of tissue-specific enolase isoenzymes may be of assistance in identifying small cell carcinomas of the lung and in distinguishing them from other pulmonary tumors. Enolase (E.C. 4.2.1.11) is a dimeric enzyme composed of various permutations of three immunologically distinct subunits alpha, beta, and gamma. Five isoenzymes alpha alpha, beta beta, gamma gamma, alpha beta, and alpha gamma have been identified. Immunohistochemical studies using antibodies to the gamma subunit have localized alpha gamma and gamma gamma specifically within neuronal and neuroendocrine tissues. Because of this limited distribution, neuron-specific enolase (NSE) can function as a biochemical marker for neuroendocrine tumors. The authors developed an enzyme-linked immunosorbent assay (ELISA) using the double antibody sandwich method. The sandwich is composed of rabbit antirat enolase that cross-reacts to the human gamma monomer, making the test specific for the gamma gamma isoenzyme. The avidin-biotin-peroxidase complex system is used to provide increased assay sensitivity. Serum samples from patients with histologically diagnosed small cell carcinoma have concentration of NSE 20- to 30-fold greater than that found in normal serum. Studies were conducted on patients with a variety of malignant pulmonary lesions and compared with controls to determine the value of NSE as a tumor marker.